(A) FIELD OF THE INVENTION
This invention relates to grinding methods and particularly relates to grinding of ceramic materials to ceramic powders. The invention especially relates to vibratory grinding of silicon carbide.
(B) HISTORY OF THE PRIOR ART
In the prior art there has been a need for silicon carbide, and other hard refractory carbides such as boron carbide, in powdered form wherein the average particle size of the powder is very small, i.e., less than about 5 microns, preferably less than 2 microns, and most preferably less than 1 micron. Such refractory carbide powders are especially required for sintering operations wherein the powders are sintered into refractory carbide articles. In the prior art, especially for silicon carbide and boron carbide which have a hardness of over 9 on the Mohs scale, it was exceedingly difficult to obtain powders having a particle size as small as desired. Furthermore it was impractical, without time consuming and expensive operation techniques, to obtain such powders where the average largest dimension (particle size) of the particles in the powders is less than 1 micron. Such powders have been obtained by sedimentation of fines from common crushing or milling operations, e.g. pure silicon carbide powder. Such methods are very inefficient, e.g. less than 1%, for the purpose of obtaining powders having average particle sizes below 1 micron. Furthermore, the grains of such powders have a generally blocky structure, e.g. an average length to width ratio of less than 2.5. Such blocky structures are believed, in accordance with the present invention and contrary to prior beliefs, to have a detrimental affect upon packing efficiency of such powders into desired shapes.
In addition, it was thought that pure silicon carbide should be used to make sinterable powders, e.g. solid solution aluminum usually less than 100 ppm and in any case less than 200 ppm. Such pure powders required costly pure starting materials which are not readily available throughout the world, e.g. pure quartz sand.
Vibratory mills in general are known in the art and, for example, are described in U.S. Pat. No. 3,268,177.
It is disclosed in SWECO, Inc. Bulletin GM781A April 1978 that alumina or zirconia cylinders could be used as media in a vibratory mill to reduce the particle size of powder. Such media is not, however, generally suitable for reducing the particle size of abrasive materials such as silicon and boron carbides due to contamination by particles from the media. Furthermore, alumina is very undesirable when the silicon carbide powder is to be used in sintering operations and cannot be easily removed from the powder. In addition, alumina is relatively dense, i.e. a specific gravity of 3.9, which requires substantial energy to vibrate alumina media.
To avoid contamination by media, it was proposed, e.g., in U.S. Pat. No. 4,275,026, to grind materials such as titanium diboride in a mill having surfaces and grinding media constructed of a noncontaminating material such as titanium diboride itself.
Use of silicon carbide as the grinding media in a vibratory mill was attempted by the inventors herein to make pure silicon carbide powders having an average particle size over 1 micron to make commercial sintered products. This method and the resulting powder were not, however, entirely satisfactory since the media had an undesirable wear rate. In addition, the silicon carbide particles resulting from media wear were exceedingly undesirable because the ultrafine powder produced and mixed with the larger particles was actually too small, e.g. an average particle size of about 0.02 microns. Even a few percent, e.g. over 5% of these fine particles have an undesirably high percentage of oxygen which unless removed by further processing, interferes with certain operations such as sintering. Even a few percent, e.g. over 5%, of such a small amount of these particles also interfere with the pressing operations used to shape an article prior to sintering. Additionally, silicon carbide media is costly and difficult to manufacture; therefore, wear of the media should be kept to a minimum.
With the exception of properties unique to silicon carbide, it is to be understood that the invention discussed herein similarly applies to other hard refractory carbides such as boron carbide.